Interpretive Summary: Administration of a CIDR 7 d before calf removal and PGF2a increased concentrations of P4 on d 0 and 18, and increased the proportion of cows exhibiting estrus. However, CIDR treatment did not improve conception and pregnancy rates in this study. Progestogens offer an advantage in causing anestrous postpartum beef cows to exhibit estrus and ovulate. Therefore, progestogens combined with PGF2a offer an effective method for synchronizing estrus in postpartum beef cows. However, this method is not effective in improving pregnancy rates, according to this study.

Technical Abstract:
Objectives for this experiment were to determine effects of a 7-d pretreatment with an intravaginal progesterone insert (CIDR) on estrous response and pregnancy rates in 3-year-old postpartum beef cows synchronized with calf removal and prostaglandin F2alpha. Cows (BW = 488 +/- 7.4; body condition score = 3.8 +/- .07; days postpartum = 58.7 +/- 1.2) were randomly allotted to either control (n = 22; i.m. injection of 25 mg PGF2alpha [Lutalyse] on d 0) or CIDR treatment from -7 to 0 d preceding, PGF2alpha injection on d 0 (CIDR; n = 18). All calves were weaned on d 0. Cows were observed for estrus for 120 h after PGF2alpha and inseminated by AI approximately 12 h after the onset of estrus. A bull was placed with cows 12 d after PGF2alpha and removed 77 d after PGF2alpha. Circulating progesterone (P4) concentrations were determined in blood samples collected on d -7, 0, and 11. Pregnancy status was diagnosed by ultrasonography on d 54 and 145 after PGF2alpha. Synchronization rates were higher (P < 0.05) for CIDR (100%) compared to Control (77%) cows. Time of estrus did not differ (P > 0.10) between Control and CIDR cows (2.41 +/- .15 d). Pregnancy rates by AI were not different (P = .28) between Control (18%) and CIDR (33%) cows. Overall pregnancy rates were higher (P < 0.10) in Control (97%) compared to CIDR (80%) cows. Concentrations of P4 on d -7, 0, and 11 did not influence (P > 0.10) overall pregnancy rates; however, progesterone concentrations were increased (P < 0.05) in CIDR cows on d 0 (5.6 vs. 2.9 ng/ml, for CIDR vs. Control) and d 11 (7.1 vs. 4.8 ng/ml, for CIDR vs. Control). Administration of a CIDR 7 d before calf removal and PGF2alpha increased concentrations of P4 on d 0 and 11, and increased the proportion of cows exhibiting estrus. However, CIDR treatment did not improve conception and pregnancy rates in this study.